This invention relates to a digital signal recording and playback apparatus, and more particularly to a digital signal recording and playback apparatus capable of satisfactorily recording or playback of a digital signal even in the case of running a magnetic tape at high speed.
For the rotary drum of a Digital Audio Tape recorder (DAT) which is an apparatus for recording and playback of a digital audio signal using a magnetic head, there is known a rotary drum in the system around which a magnetic tape (hereinafter simply referred to as a "tape") 3 is wound in an angular range of 90 degrees of a rotary drum 2 on which two rotary heads 1a and 1b having different azimuth angles are provided at an angular interval of 180 degrees as shown in FIGS. 1A and 1B.
In these figures, reference numerals 41 to 44 denote guide rods for guiding the tape 3 for the purpose of allowing the tape 3 to be in contact with the rotary drum 2, respectively.
There is an established Industry Standard for the DAT (which may be referred to as simply "the DAT format" in this specification) which specifies or recommends specifications of tape format and of related mechanical components and their performances. According to the, standard, track length L of 2.3501 cm, a track angle .theta. of 6 degrees 22 minutes 59.5 seconds, and a tape transport speed of 0.815 cm/sec are standardized, and a drum diameter of 30 mm and drum rotational speed of 2,000 revolutions per minute (r.p.m.) are recommended for one of the various operating modes. And a lead angle of 6 degrees 22 minutes, which will be explained later, is recommended also.
For a convenience of explanation of this invention, all of those standardized or recommended figures in the DAT format are referred to as "standard" values in this specification, further, recording and reproducing operation under those "standard" values is referred to as "standard" mode unless otherwise specified.
The above-mentioned rotary drum 2 has the standard diameter of 30 mm and rotates at the standard rotational speed of 2000 revolutions per minute (hereinafter referred to as 2000 r.p.m), and the tape 3 is transported at the standard speed of 0.815.multidot.cm/sec. Furthermore, the lead or introduction angle when the tape 3 is wound onto the rotary drum 3 is the standard lead angle 6 degrees 22 minutes (hereinafter referred to as 6.degree.', etc.). The lead angle is the inclination of the rotary axis of the rotary drum with respect to the perpendicularly transversed direction of the magnetic tape obliquely wrapped around the rotary drum. The relationship between the rotational direction R of the rotary drum 2 and the running direction S of the magnetic tape 3 is actually determined as shown in FIG. 1. As a result, at the time of recording, the digital signal is recorded while forming tracks as a head scanning locus 4a and 4b on the tape 3 as shown in FIG. 2, and a track angle .theta. defined by these tracks and the running direction S of the tape 3 is equal to 6.degree.22'59.5" standardized in the DAT format.
On the other hand, at the time of playback, the magnetic heads 1a and 1b provided and being angularly spaced to each other by an angle of 180 degrees alternately scan these tracks 4a and 4b consecutively formed without a guard band therebetween, whereby the recorded digital signal is read out. At this time, the time required until the magnetic head 1a or 1b rotates by an angle of 90 degrees to scan one track is 7.5 milliseconds (msec).
In such an arrangement where the tape 3 is wound in an angular range of 90 degrees onto the rotary drum 2 on which the magnetic heads 1a and 1b are provided at an interval of 180 degrees, there is a 7.5 msec blank period during which the tape is not contacted nor scanned by either magnetic head 1a or 1b, between the end of one track scan by one head and the beginning of the subsequent track scan by another head. Accordingly, in order to adapt the continuous recording or playback digital signal to such an intermittent operation which is carried out only when the magnetic head is scanning the track, time compression processing is implemented to the digital signal.
In general, the time required for dubbing a digital audio signal from one DAT to another DAT having a rotary drum constituted as shown in FIG. 1 is equal to the time required for reproducing the signal on the tape at the standard speed. For example, in the case of dubbing a two hour length of digital program, it takes two hours which is the same as that at the time of playback for normal listening. On the other hand, most tape recorders of analog system so-called "doubledeck machines" have a function capable of carrying out dubbing at double speed or a speed higher than a standard one. Under these circumstances, a high speed dubbing function is desired also for DATs.
Furthermore, in the case of using DAT for backing up a hard disk, etc. for use in a computer system as a data streamer, or in other similar cases, there is a need for carrying out recording/playback of data at a high speed. The transfer speed in this case is restricted by the basic electrical and mechanical characteristics of the DAT.
If an attempt was made to carry out a dubbing, e.g., at a quadruple tape speed, i.e. 4 times the standard tape speed, using the apparatus of FIG. 1, it would require to rotate the rotary drum at 8000 r.p.m. which is four times faster than the standard speed. However, since the relative speed between the magnetic head 1a or 1b scanning the tracks 4a and 4b on the tape 3 and the tape 3 would also become equal to a value four times as large, the recording/playback frequencies become extremely high. As a result, the signal level would be lowered by the inherent characteristics of the magnetic head 1a or 1b and the rotary transformer (not shown) provided within the rotary drum 2 to conduct transmission and reception of signals, resulting in the problem that it would be unable to carry out recording and playback in full fidelity. Furthermore, at this time, the modulator circuit and the demodulator circuit, etc. must perform circuit operation at four times the speed.
In addition, there would be a problem of revolving the rotary drum 2 at 8000 r.p.m. which is a value four times larger than the standard value. There would likely develop unstable tape running in the tape transport system.